Thermal property sensors of the type described herein have a wide range of applications (e.g. detectors in gas-liquid chromatographs (GLC), flow monitors or liquid level detectors), and the basic concepts of their operation are well-known in prior art. A resistive element is heated and the resistance of the element is monitored to determine changed heat transfer conditions at the element. For example, if the medium surrounding the element changes from a gas to a liquid and becomes a better heat sink, much of the heat will be conducted away from the element, and the temperature of the element will decrease. This leads to a decrease in the resistance of the element which may be detected as an increase in the current through or a decrease in the voltage across the element or both.
Changes in the thermal properties of the medium such as thermal conductivity changes (e.g. a change in the composition of a GLC sample or a change in the level of a liquid) may be sensed by monitoring the changes in the current through and/or the voltage across the element. This works well in systems where the temperature of the medium remains constant.
Changes in the temperature of the medium will cause the temperature of the element to change as well. For example, a decrease in ambient temperature of the medium will decrease the resistance of the element and, as a consequence, the current will decrease or the voltage will increase. This change in ambient temperature may be misinterpreted as an increase in the thermal conductivity of the medium (i.e. the presence of a component in a GLC sample or an increase in the level of a liquid).
There are a number of attempts in the prior art to solve this problem. In gas-liquid chromatography, the carrier gas stream is apportioned into two paths prior to insertion of the sample, and a temperature reference is obtained from the second stream for providing temperature compensation. This ability to compensate for variations in temperature doubles the amount of gas needed for the chromatogram.
When measuring the level of a liquid, the prior art has resorted to the use of either a plurality of detectors or a temperature monitor in combination with a computer to compensate for the changes (such as is disclosed in U.S. Pat. No. 4,590,797, issued May 27, 1986 to Beaubatie, et al.). Others have assumed that there is no significant change in the temperature (such as is disclosed in U.S. Pat. No. 3,302,458, issued Feb. 7, 1967 to Scadron).